1. Field of the Invention
The present invention relates to a motor spindle for a hard disc device which is utilized for rotatably driving a hard disk or a read-out arm.
2. Description of the Prior Art
FIG. 1 and FIG. 2 show a first example of a conventional motor spindle for driving a hard disc or disk. Conventionally, a motor spindle such as, for example, that shown in FIG. 1 is incorporated in a hard disk device (HDD) having a motor to rotatably drive a hard disk or a motor to drive a read-out arm.
This motor spindle is provided with a housing 2 which is secured to a mounting base or the like by means of a mounting flange 1 formed on the outer peripheral surface of the housing 2. A rotatable shaft 3 is supported in a freely rotatable manner on the inside of the housing 2 through a pair of ball bearings 4. A rotor 5 is secured to one end of the rotatable shaft 3, and a hub 6 is secured to the other end of the rotatable shaft 3. A hard disk (omitted from the drawing) is secured to the hub 6. Since precisely controlled rotation is required for motors used in the hard disk device, the rotatable shaft 3 is supported by the pair of bearings 4, where a predetermined pressure is applied to between the pair of bearings 4 to eliminate chattering.
The structure of the ball bearings 4 is illustrated in FIG. 2. The ball bearings 4 have outer and inner rings 7 and 9, respectively. An inward-facing track 8 is provided on the inner peripheral surface of the outer ring 7; an outward-facing track 10 is provided on the outer peripheral surface of the inner ring 9. A plurality of balls 11 are provided between the inward-facing track 8 and the outward-facing track 10. A retainer 12 is incorporated in the bearing 4 to maintain the balls 11 therein in a freely rotatable state. A pair of seal plates 13 are also incorporated in the bearing 4. The outer peripheral edges of the pair of seal plates 13 are supported on an inner peripheral surface at each end of the outer ring 7; the inner peripheral edges of the seal plates 13 extend close to the outer peripheral surface of the inner ring 9 at each end thereof.
Incidentally, the terms "inward", "outward", "inner" and "outer" are used to indicate radial directions around the shaft 3 in this specification.
The ball bearings 4 are installed in the motor spindle for driving the hard disk such that the outer ring 7 of each of the ball bearings 4 is interposedly secured in or fitted into the housing 2 of the motor spindle, and the inner ring 9 is arranged to fixedly enclose the rotatable shaft 3 or fitted onto the rotatable shaft 3. As a result of this configuration, the rotatable shaft 3 is supported in a freely rotatable manner on the inside of the housing 2.
In addition, the seal plates 13 supported on the inner peripheral surfaces of the outer ring 7 at each side thereof act to prevent leakage of grease which is present in the interior between the two seal plates 13.
In the ball bearings 4 as mentioned above, at least one of the pair of seal plates 13 (the upper seal plate of the upper bearing 4 and the lower seal plate of the lower bearing 4 in FIG. 2) must be provided at a position considerably separated from the balls 11 which are supported by the retainer 12. Specifically, an annular main section of the retainer 12 is positioned between the balls 11 and the at least one of the seal plates 13. Therefore, to prevent interference between the at least one of the seal plates 13 and the retainer 12 (the annular main section mentioned above), it is necessary that the balls 11 and the at least one of the seal plates 13 be sufficiently separated. Accordingly, it is difficult to fabricate the ball bearing 4 with a small width W (FIG. 2), and therefore to make the axial size T (FIG. 1) of the motor spindle incorporating this type of ball bearing small to obtain a thin motor spindle.
With the increasing popularity of lap-top models of OA equipment in recent years, the space available for installing an HDD and the like is extremely limited. Accordingly, there are strong demands for even a small reduction (for example, as small as 1 mm) in the axial size T of the motor spindle.
FIG. 3 shows a second example of a conventional motor spindle for driving a hard disk.
Conventionally, a motor or the like for driving, for example, a windshield wiper of an automobile incorporates a ball bearing of the type illustrated in FIG. 3 (see, for example, U.S. Pat. No. 3,554,621, Japanese Utility Model Applications No. S50-148441 and No. S55-119426).
This ball bearing comprises an outer ring 7, an inner ring 9, a plurality of balls 11 provided between the outer ring 7 and the inner ring 9 and a retainer 14 provided on one side of the ball bearing.
An inward-facing track 8 is formed on the inner peripheral surface of the outer ring 7; an outward-facing track 10 is formed on the outer peripheral surface of the inner ring 9; and the plurality of balls 11 are slidably supported by the retainer 14, the outer ring 7 and the inner ring 9. The retainer 14 is formed in a circular ring shape from a synthetic resin material.
A flange-shaped first seal plate 15 is provided integrally with the retainer 14 on one side thereof (the upper side in FIG. 3). The outer peripheral edge and the inner peripheral edge of the first seal plate 15 are positioned adjacent to the inner peripheral surface of the outer ring 7 and the outer peripheral surface of the inner ring 9, respectively.
The ball bearing is also provided with a second seal plate 17 which is formed in a hoop shape from a metal plate. The outer peripheral edge of the second seal plate 17 is supportedly secured to the inner peripheral surface at the other end of the outer ring 7 remote from the retainer 14.
The ball bearing shown in FIG. 3 is characterized in a compact structure, but not so good in sealing grease, and therefore, is suitable for a windshield wiper of an automobile and the like where precise sealing is not required so long as grease is sufficiently retained. However, this bearing could not be expected to be useful in a hard disk device the space for the hard disk of which must be kept in an extremely clean condition. The reason for this is that even a slight amount of grease adheres to the hard disk, specifically a magnetic recording disk to become dusts in the extremely clean space causing read-out errors.